Tire rubber composition and pneumatic tire

ABSTRACT

Provided are a rubber composition for tires preventing discoloration and improving ozone resistance while maintaining or improving good elongation at break and good fuel economy, and a pneumatic tire using the rubber composition. Included is a rubber composition for tires including: a rubber component including at least one selected from the group consisting of a specific non-oil extended polybutadiene rubber and a specific oil extended polybutadiene rubber; a phenylenediamine antioxidant; a specific nonionic surfactant; and carbon black and/or silica, the rubber composition having an amount of diene rubber of 70-100% by mass per 100% by mass of the rubber component, the rubber composition having an amount of the phenylenediamine antioxidant of 1.0-8.0 parts by mass, an amount of the nonionic surfactant of 0.1-5.0 parts by mass, and a combined amount of carbon black and silica of 20-45 parts by mass, each per 100 parts by mass of the rubber component.

TECHNICAL FIELD

The present invention relates to a rubber composition for tires and apneumatic tire using the rubber composition.

BACKGROUND ART

Since automobile tires are manufactured using rubber compositions madefrom natural rubber and/or synthetic diene rubbers, degradation of suchtires is accelerated at high oxygen or ozone concentrations or underultraviolet rays, which may result in the formation of cracks. In orderto suppress crack formation and growth in the presence of ozone, forexample, additives such as antioxidants, e.g.,N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) orpoly(2,2,4-trimethyl-1,2-)dihydroquinoline (TMDQ), or petroleum wax areused in rubber compositions.

The antioxidants and petroleum wax in rubber vulcanizates migrate(bloom) to the rubber surface of, for example, tires, thereby serving toprotect the rubbers from ozone. Unfortunately, excessive blooming of theantioxidants and petroleum wax in a short period of time causes whitediscoloration. Moreover, the antioxidants oxidized by ozone cause browndiscoloration, and similar excessive blooming of them intensifies browndiscoloration. Furthermore, if the wax and the like bloomed on the tiresurface form an uneven bloom layer (surface-protecting layer), diffusereflection of light occurs, making the brown discoloration caused by thedegraded antioxidants more noticeable. Thus, it has been difficult toimprove ozone resistance while preventing discoloration.

Patent Literature 1 describes that the addition of a polyoxyethyleneether nonionic surfactant prevents deterioration of the appearance oftires. This technique still leaves room for improvement in terms ofpreventing discoloration and improving ozone resistance whilemaintaining or improving good elongation at break and good fuel economy.

CITATION LIST Patent Literature

Patent Literature 1: JP H05-194790 A

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problem and provide arubber composition for tires capable of preventing discoloration andimproving ozone resistance while maintaining or improving goodelongation at break and good fuel economy, and a pneumatic tire usingthe rubber composition.

Solution to Problem

The present invention relates to a rubber composition for tires,containing:

a rubber component including at least one selected from the groupconsisting of a non-oil extended polybutadiene rubber that has a ciscontent of 95% by mass or more and a Mooney viscosity (ML₁₊₄, 100° C.)of 50 or more and an oil extended polybutadiene rubber that has a ciscontent of 95% by mass or more and a weight average molecular weight(Mw) of 420,000 or higher;

a phenylenediamine antioxidant;

a nonionic surfactant; and

at least one of carbon black or silica,

the non-oil extended polybutadiene rubber being at least one selectedfrom the group consisting of a polybutadiene rubber synthesized using arare earth catalyst and a polybutadiene rubber containing1,2-syndiotactic polybutadiene crystals,

the nonionic surfactant being at least one selected from the groupconsisting of a Pluronic-type nonionic surfactant and at least one ofnonionic surfactants represented by Formula (1) or Formula (2) below,

the rubber composition having an amount of diene rubber of 70 to 100% bymass based on 100% by mass of the rubber component,

the rubber composition having an amount of the phenylenediamineantioxidant of 1.0 to 8.0 parts by mass, an amount of the nonionicsurfactant of 0.1 to 5.0 parts by mass, and a combined amount of carbonblack and silica of 20 to 45 parts by mass, each per 100 parts by massof the rubber component,

wherein R¹ represents a C6-C26 hydrocarbon group, and d represents aninteger,

wherein R² and R³ are the same or different and each represent a C6-C26hydrocarbon group, and e represents an integer.

Preferably, the rubber composition for tires contains, based on 100% bymass of the rubber component, 15 to 70% by mass of the non-oil extendedpolybutadiene rubber or 5 to 50% by mass of a polybutadiene rubbercomponent contained in the oil extended polybutadiene rubber, orcontains, based on 100% by mass of the rubber component, 15 to 70% bymass of the non-oil extended polybutadiene rubber and 5 to 50% by massof a polybutadiene rubber component contained in the oil extendedpolybutadiene rubber.

The non-oil extended polybutadiene rubber preferably has a Mooneyviscosity (ML₁₊₄, 100° C.) of 60 or more.

Preferably, an amount of petroleum-derived wax is 6.0 parts by mass orless per 100 parts by mass of the rubber component.

Preferably, based on 100% by mass of the petroleum-derived wax, acombined amount of C20 to C32 normal alkanes is 25 to 50% by mass and acombined amount of C33 to C44 normal alkanes is 25 to 50% by mass.

The rubber composition for tires is preferably a rubber composition fortire outer layers.

The present invention also relates to a pneumatic tire, formed from therubber composition.

Advantageous Effects of Invention

The present invention provides a rubber composition for tirescontaining: a rubber component including at least one selected from thegroup consisting of a specific non-oil extended polybutadiene rubberthat has a cis content of 95% by mass or more and a Mooney viscosity(ML₁₊₄, 100° C.) of 50 or more and an oil extended polybutadiene rubberthat has a cis content of 95% by mass or more and a weight averagemolecular weight (Mw) of 420,000 or higher, the rubber componentincluding a predetermined amount of diene rubber; a predetermined amountof a phenylenediamine antioxidant; a predetermined amount of a specificnonionic surfactant; and a predetermined amount of carbon black and/orsilica. Such a rubber composition for tires is capable of preventingdiscoloration and improving ozone resistance while maintaining orimproving good elongation at break and good fuel economy.

DESCRIPTION OF EMBODIMENTS

The rubber composition for tires of the present invention contains: arubber component including at least one selected from the groupconsisting of a specific non-oil extended polybutadiene rubber that hasa cis content of 95% by mass or more and a Mooney viscosity (ML₁₊₄, 100°C.) of 50 or more and an oil extended polybutadiene rubber that has acis content of 95% by mass or more and a weight average molecular weight(Mw) of 420,000 or higher, the rubber component including apredetermined amount of diene rubber; a predetermined amount of aphenylenediamine antioxidant; a predetermined amount of a specificnonionic surfactant; and a predetermined amount of carbon black and/orsilica. Such a rubber composition can exhibit excellent ozone resistanceover a wide temperature range and at the same time can be sufficientlyprevented from suffering brown discoloration and white discoloration onthe tire surface, so that these properties are simultaneously ensured.The rubber composition for tires of the present invention has bothdiscoloration resistance and ozone resistance while maintaining orimproving good elongation at break and good fuel economy as describedabove, presumably for the following reasons.

The specific surfactant blooms to the tire surface together with wax andantioxidants and melts and flattens them, as a result of which whitediscoloration can be diminished and, at the same time, theirregularities of the surface-protecting layer formed on the tiresurface are reduced so that brown discoloration, which is noticeableunder diffuse reflection, can be greatly diminished. Further, a shinyblack luster is imparted to the tire surface, and ozone resistance isalso improved. However, although the addition of the specific surfactantenables discoloration resistance and ozone resistance to be ensuredsimultaneously, it creates the following new problems: The rubbercomposition has a reduced viscosity and exhibits increased lubricityduring kneading, which makes it difficult to transmit the torque of thekneading machine so as to disperse filler. As a result, silica andcarbon black are less dispersed so that fuel economy and elongation atbreak are reduced. In order to solve the problems, the rubbercomposition of the present invention contains a high viscositypolybutadiene rubber such as the specific non-oil extended polybutadienerubber or the specific oil extended polybutadiene rubber together withthe specific surfactant. Such a rubber composition has bothdiscoloration resistance and ozone resistance while maintaining orimproving good elongation at break and good fuel economy, and thus canexhibit excellent ozone resistance over a wide temperature range and atthe same time can sufficiently suppress discoloration. Moreover, whensuch a polybutadiene rubber having a relatively high molecular weight asthe specific non-oil extended polybutadiene rubber or the specific oilextended polybutadiene rubber is used, the number of polymer chain endsis reduced and the energy loss due to their free movement decreases; inaddition, the polymer can easily be tightly entangled with otherpolymers or filler and thus is less likely to be unfastened therefrom.As a result, good fuel economy, good elongation at break, and good crackgrowth resistance are achieved.

According to the present invention, the rubber component includes atleast one selected from the group consisting of a non-oil extendedpolybutadiene rubber having a cis content of 95% by mass or more and aMooney viscosity (ML₁₊₄, 100° C.) of 50 or more (non-oil extendedpolybutadiene rubber) and an oil extended polybutadiene rubber having acis content of 95% by mass or more and a weight average molecular weight(Mw) of 420,000 or higher (oil extended polybutadiene rubber).

The oil extended polybutadiene rubber refers to a rubber prepared byadding an extending component such as oil to polybutadiene rubber in theproduction of the polymer. The non-oil extended polybutadiene rubberrefers to a rubber prepared without adding an extending component topolybutadiene rubber.

First, the non-oil extended polybutadiene rubber will be described. Thenon-oil extended polybutadiene rubber has a cis content of 95% by massor more and a Mooney viscosity (ML₁₊₄, 100° C.) of 50 or more, and is atleast one selected from the group consisting of a polybutadiene rubbersynthesized using a rare earth catalyst and a polybutadiene rubbercontaining 1,2-syndiotactic polybutadiene crystals.

The non-oil extended polybutadiene rubber has a cis content of 95% bymass or more, preferably 96% by mass or more. If the non-oil extendedpolybutadiene rubber has a cis content of less than 95% by mass, goodabrasion resistance or good elongation at break cannot be obtained. Themaximum cis content is not particularly limited.

The non-oil extended polybutadiene rubber has a Mooney viscosity (ML₁₊₄,100° C.) of 50 or more, preferably 60 or more. If the non-oil extendedpolybutadiene rubber has a Mooney viscosity of less than 50, the rubbercomposition cannot simultaneously achieve discoloration resistance andozone resistance while maintaining or improving good elongation at breakand good fuel economy. Moreover, the maximum Mooney viscosity is notparticularly limited, but is preferably 80 or less, more preferably 70or less. If the Mooney viscosity is more than 80, the polymer is lesslikely to be dispersed and also less likely to incorporate filler, withthe result that crack growth resistance and elongation at break tend tobe deteriorated.

The Mooney viscosity (ML₁₊₄, 100° C.) is determined by measuring Mooneyviscosity at 100° C. in conformity with JIS K 6300.

The non-oil extended polybutadiene rubber preferably has a highmolecular weight to satisfy the above Mooney viscosity. Specifically,the non-oil extended polybutadiene rubber preferably has a weightaverage molecular weight (Mw) of 400,000 or higher, more preferably500,000 or higher, still more preferably 560,000 or higher. Moreover,the maximum Mw is not particularly limited, but is preferably 900,000 orlower, more preferably 700,000 or lower. If the Mw is higher than900,000, the polymer is less likely to be dispersed and also less likelyto incorporate filler, with the result that crack growth resistance andelongation at break tend to be deteriorated.

The non-oil extended polybutadiene rubber preferably contains a largeamount of 1,2-syndiotactic polybutadiene crystals (SPB) to satisfy theabove Mooney viscosity. Specifically, the SPB content in the non-oilextended polybutadiene rubber is preferably 14% by mass or more, morepreferably 16% by mass or more. Moreover, the maximum SPB content is notparticularly limited, but is preferably 21% by mass or less, morepreferably 20% by mass or less. If the SPB content is more than 21% bymass, the polymer is less likely to be dispersed and also less likely toincorporate filler, with the result that crack growth resistance andelongation at break tend to be deteriorated.

Since SPB-containing BR has a high viscosity due to the bonding of SPBto the BR matrix, BR containing not less than a specific amount of SPBsatisfies the above Mooney viscosity.

Thus, the non-oil extended polybutadiene rubber is preferably a BRhaving a weight average molecular weight within the above range or a BRcontaining not less than a specific amount of SPB. The BR having aweight average molecular weight within the above range is alsopreferably a BR synthesized using a rare earth catalyst (rareearth-catalyzed BR). The rare earth-catalyzed BR has a high cis contentand a low vinyl content and therefore not only has good abrasionresistance but also provides good fuel economy, good elongation atbreak, and good crack growth resistance.

In a preferred embodiment of the SPB-containing BR, in view of abrasionresistance and extrusion processability, and in order to easily satisfythe above Mooney viscosity, SPB crystals are not merely dispersed in BRbut are chemically bonded to BR and dispersed therein. The SPBpreferably has a melting point of 180° C. to 220° C. The SPB content inthe SPB-containing BR is preferably 2.5 to 20% by mass. The SPB contentin the SPB-containing BR refers to the amount of boiling n-hexaneinsolubles.

In the case of the rubber composition of the present inventioncontaining SPB-containing BR as the non-oil extended polybutadienerubber, the amount of SPB-containing BR based on 100% by mass of therubber component is preferably 5% by mass or more, more preferably 10%by mass or more. The amount is preferably 70% by mass or less, morepreferably 60% by mass or less, still more preferably 40% by mass orless. When the amount of SPB-containing BR is within the above range,the effects of the present invention can be more suitably achieved.

Next, the rare earth-catalyzed BR will be described.

The rare earth-catalyzed BR refers to a polybutadiene rubber synthesizedusing a rare earth catalyst, and features a high cis content and a lowvinyl content. As the rare earth-catalyzed BR, those commonly used inthe production of tires can be used.

The rare earth catalyst may be a known one and examples includecatalysts containing lanthanide rare earth compounds, organic aluminumcompounds, aluminoxanes, or halogen-containing compounds, optionallywith Lewis bases. Among these, Nd catalysts are especially preferredwhich contain neodymium (Nd)-containing compounds as lanthanide rareearth compounds.

Examples of the lanthanide rare earth compounds include halides,carboxylates, alcholates, thioalcholates, and amides of rare earthmetals of atomic numbers 57 to 71. As described above, Nd catalysts arepreferred among these because they allow the resulting BR to have a highcis content and a low vinyl content.

Examples of the organic aluminum compounds include compounds representedby AlR^(a)R^(b)R^(c), where R^(a), R^(b) and R^(c) are the same ordifferent and each represent hydrogen or a hydrocarbon group having 1 to8 carbon atoms. Examples of the aluminoxanes include acyclicaluminoxanes and cyclic aluminoxanes. Examples of the halogen-containingcompounds include aluminum halides represented by AlX_(k)R^(d) _(3−k),where X represents a halogen; R^(d) represents a C1-20 alkyl, aryl oraralkyl group; and k represents 1, 1.5, 2 or 3; strontium halides suchas Me₃SrCl, Me₂SrCl₂, MeSrHCl₂, and MeSrCl₃; and metal halides such assilicon tetrachloride, tin tetrachloride, and titanium tetrachloride.Lewis bases are used for complexation of lanthanide rare earthcompounds, and suitable examples include acetylacetone, ketones andalcohols.

In butadiene polymerization, the rare earth catalyst may be used insolution in an organic solvent (e.g. n-hexane, cyclohexane, n-heptane,toluene, xylene, or benzene) or may be supported on an appropriatecarrier, such as silica, magnesia, or magnesium chloride. Regarding thepolymerization conditions, the polymerization may be either solutionpolymerization or bulk polymerization, and the polymerizationtemperature is preferably −30° C. to 150° C., and the polymerizationpressure may be appropriately chosen depending on other conditions.

The rare earth-catalyzed BR preferably has a ratio (Mw/Mn) of the weightaverage molecular weight (Mw) to the number average molecular weight(Mn) of 1.2 or higher, more preferably 1.5 or higher. At a ratio oflower than 1.2, processability tends to be markedly deteriorated. Theratio Mw/Mn is preferably 5.0 or lower, more preferably 4.5 or lower,still more preferably 4.0 or lower, particularly preferably 3.5 orlower. At a ratio of higher than 5.0, the effect of improving abrasionresistance tends to be reduced.

The rare earth-catalyzed BR preferably has a vinyl content of 1.8% bymass or less, more preferably 1.0% by mass or less, still morepreferably 0.9% by mass or less. If the rare earth-catalyzed BR has avinyl content of more than 1.8% by mass, abrasion resistance may bereduced. The minimum vinyl content is not particularly limited.

In the case of the rubber composition of the present inventioncontaining rare earth-catalyzed BR as the non-oil extended polybutadienerubber, the amount of rare earth-catalyzed BR based on 100% by mass ofthe rubber component is preferably 5% by mass or more, more preferably10% by mass or more. The amount is preferably 70% by mass or less, morepreferably 60% by mass or less, still more preferably 40% by mass orless. When the amount of rare earth-catalyzed BR is within the aboverange, the effects of the present invention can be more suitablyachieved.

Examples of the non-oil extended polybutadiene rubber include VCR617(SPB-containing BR, SPB content: 17% by mass, cis content: 96% by mass,Mooney viscosity (ML₁₊₄, 100° C.): 62) available from Ube Industries,Ltd., BUNA-CB22 (rare earth-catalyzed BR synthesized using a Ndcatalyst, Mw: 590,000, cis content: 97% by mass, Mooney viscosity(ML₁₊₄, 100° C.): 63) available from LANXESS, and BR730 (rareearth-catalyzed BR synthesized using a Nd catalyst, Mw: 580,000, ciscontent: 97% by mass, Mooney viscosity (ML₁₊₄, 100° C.): 51) availablefrom JSR Corporation.

In the case of the rubber composition of the present inventioncontaining the non-oil extended polybutadiene rubber, the amount of thenon-oil extended polybutadiene rubber based on 100% by mass of therubber component is preferably 5% by mass or more, more preferably 15%by mass or more. The amount is preferably 70% by mass or less, morepreferably 60% by mass or less, still more preferably 40% by mass orless. When the amount of the non-oil extended polybutadiene rubber iswithin the above range, the effects of the present invention can be moresuitably achieved.

Next, the oil extended polybutadiene rubber will be described. The oilextended polybutadiene rubber has a cis content of 95% by mass or moreand a weight average molecular weight (Mw) of 420,000 or higher.

The oil extended polybutadiene rubber has a cis content of 95% by massor more, preferably 96% by mass or more. If the oil extendedpolybutadiene rubber has a cis content of less than 95% by mass, goodabrasion resistance or good elongation at break cannot be obtained. Themaximum cis content is not particularly limited.

The oil extended polybutadiene rubber has a weight average molecularweight (Mw) of 420,000 or higher, preferably 450,000 or higher, morepreferably 500,000 or higher, still more preferably 600,000 or higher,particularly preferably 700,000 or higher. Moreover, the maximum Mw isnot particularly limited, but is preferably 1,000,000 or lower, morepreferably 950,000 or lower, still more preferably 900,000 or lower. Ifthe oil extended polybutadiene rubber has a Mw of higher than 1,000,000,the polymer is less likely to be dispersed and also less likely toincorporate filler, with the result that crack growth resistance andelongation at break tend to be deteriorated.

Herein, the cis content (cis-1,4-butadiene unit content) and the vinylcontent (1,2-butadiene unit content) in BR can be determined by infraredabsorption spectrometry. The weight average molecular weight (Mw) andthe number average molecular weight (Mn) of BR are measured using a gelpermeation chromatograph (GPC) (GPC-8000 series available from TosohCorporation, detector: differential refractometer, column: TSKGELSUPERMULTIPORE HZ-M available from Tosoh Corporation) and calibratedwith polystyrene standards.

In the oil extended polybutadiene rubber, the amount of the extendingcomponent per 100 parts by mass of the rubber component is, for example,but not limited to, 10 to 50 parts by mass.

The oil extended polybutadiene rubber is preferably a BR synthesizedusing a rare earth catalyst (rare earth-catalyzed BR). The rareearth-catalyzed BR has a high cis content and a low vinyl content andtherefore not only has good abrasion resistance but also provides goodfuel economy, good elongation at break, and good crack growthresistance. The rare earth-catalyzed BR may be a similar suitableembodiment as mentioned for the earlier described rare earth-catalyzedBR.

In the case of the rubber composition of the present inventioncontaining rare earth-catalyzed BR as the oil extended polybutadienerubber, the amount of rare earth-catalyzed BR based on 100% by mass ofthe rubber component is preferably 5% by mass or more, more preferably15% by mass or more. The amount is preferably 50% by mass or less, morepreferably 40% by mass or less, still more preferably 35% by mass orless. When the amount of rare earth-catalyzed BR is within the aboverange, the effects of the present invention can be more suitablyachieved.

Examples of the oil extended polybutadiene rubber include BUNA-CB29 TDAE(rare earth-catalyzed BR synthesized using a Nd catalyst, containing37.5 parts by mass of TDAE per 100 parts by mass of the rubbercomponent; cis content: 97% by mass, Mw: 760,000, Mn: 326,000) availablefrom LANXESS and BUNA-CB29 MES (rare earth-catalyzed BR synthesizedusing a Nd catalyst, containing 37.5 parts by mass of MES per 100 partsby mass of the rubber component; cis content: 97% by mass, Mw: 737,000,Mn: 191,000) available from LANXESS.

In the case of the rubber composition of the present inventioncontaining the oil extended polybutadiene rubber, the amount of thepolybutadiene rubber component contained in the oil extendedpolybutadiene rubber, based on 100% by mass of the rubber component ispreferably 5% by mass or more, more preferably 15% by mass or more. Theamount is preferably 50% by mass or less, more preferably 40% by mass orless, still more preferably 35% by mass or less. When the amount of theoil extended polybutadiene rubber satisfies the above range, the effectsof the present invention can be more suitably achieved.

In addition to the non-oil extended polybutadiene rubber and/or the oilextended polybutadiene rubber, BRs other than the non-oil extendedpolybutadiene rubber and the oil extended polybutadiene rubber (otherBRs) may be used in the present invention. Non-limiting examples ofother BRs include those commonly used in the tire industry, such ashigh-cis content BR, for example, BR1220 available from ZeonCorporation, BR150B available from Ube Industries, Ltd. or the like, BRcontaining 1,2-syndiotactic polybutadiene crystals (SPB), for example,VCR412 available from Ube Industries, Ltd., and polybutadiene rubbersynthesized using a rare earth catalyst (rare earth-catalyzed BR). Otherexamples include modified polybutadiene rubbers such as tin-modifiedpolybutadiene rubber (tin-modified BR (modified BR for carbon black))which has been modified with a tin compound, e.g., tin-modified BRpolymerized using a lithium initiator and having a vinyl bond content of5 to 50% by mass, an Mw/Mn of 2.0 or less, and a tin atom content of 50to 3000 ppm. Among these, tin-modified BR is preferred.

Preferably, the tin-modified BR is prepared by polymerization of1,3-butadiene using a lithium initiator, followed by the addition of atin compound, and has a tin-carbon bond at a molecular chain endthereof.

Examples of the lithium initiator include lithium compounds such asalkyllithiums and aryllithiums. Examples of the tin compound include tintetrachloride and butyltin trichloride. The tin-modified BR preferablyhas a tin atom content of 50 to 3000 ppm. The tin-modified BR preferablyhas a molecular weight distribution (Mw/Mn) of 2 or less. Moreover, thetin-modified BR preferably has a vinyl content of 5 to 50% by mass.

In the case of the rubber composition of the present inventioncontaining tin-modified BR, the amount of tin-modified BR based on 100%by mass of the rubber component is preferably 5% by mass or more, morepreferably 15% by mass or more. The amount is preferably 50% by mass orless, more preferably 40% by mass or less, still more preferably 35% bymass or less. When the amount of tin-modified BR is within the aboverange, the effects of the present invention can be more suitablyachieved.

Examples of a rubber component usable in the present invention otherthan BR include diene rubbers such as isoprene-based rubbers,styrene-butadiene rubber (SBR), styrene-isoprene-butadiene rubber(SIBR), chloroprene rubber (CR), or acrylonitrile-butadiene rubber(NBR); and non-diene rubbers such as ethylene-propylene-diene rubber(EPDM), butyl rubber (IIR) or halogenated butyl rubber (X-IIR). Each ofthese may be used alone, or two or more of these may be used incombination. Among these, diene rubbers are preferred because they canbe suitably used for tires. Further, isoprene-based rubbers and SBR arepreferred among diene rubbers because they provide good durability whileensuring good handling stability, good fuel economy, and good elongationat break. More preferred are isoprene-based rubbers. For use insidewalls or clinches, isoprene-based rubbers are preferred for goodtensile strength. For use in treads, SBR is preferred for excellent gripperformance.

The amount of diene rubber based on 100% by mass of the rubber componentis 70% by mass or more, preferably 80% by mass or more, more preferably90% by mass or more, and may be 100% by mass. The rubber compositioncontaining the above amount of diene rubber can suitably enjoy theeffects of the present invention and can also be suitably used as arubber composition for tires.

Examples of isoprene-based rubbers include synthetic polyisoprene rubber(IR), natural rubber (NR), and modified natural rubber. Examples of NRinclude deproteinized natural rubber (DPNR) and high purity naturalrubber (HPNR). Examples of modified natural rubber include epoxidizednatural rubber (ENR), hydrogenated natural rubber (HNR), and graftednatural rubber. Moreover, the NR may be one commonly used in the tireindustry, for example, SIR20, RSS#3, TSR20, or the like. Among these, NRor IR is preferred, and NR is more preferred.

The amount of isoprene-based rubber based on 100% by mass of the rubbercomponent is preferably 10 to 80% by mass. In this case, good crackgrowth resistance and good mechanical strength can be obtained.

In the case of the rubber composition of the present invention for usein sidewalls or clinches, the amount of isoprene-based rubber based on100% by mass of the rubber component is preferably 20% by mass or more,more preferably 30% by mass or more. If the amount is less than 20% bymass, sufficient mechanical strength may not be obtained. The amount ofisoprene-based rubber is preferably 80% by mass or less, more preferably70% by mass or less. If the amount is more than 80% by mass, crackgrowth resistance and the like may be reduced. Also in the case of therubber composition of the present invention for use in treads or thelike, a similar range as above may be used. The amount of isoprene-basedrubber may be appropriately varied depending on whether the rubbercomposition is for use in passenger vehicles or in trucks and buses.

The combined amount of isoprene-based rubber and BR based on 100% bymass of the rubber component is preferably 90% by mass or more, morepreferably 95% by mass or more, and may be 100% by mass. In such case,good handling stability, good fuel economy, good elongation at break,good abrasion resistance, good durability, and good crack growthresistance can be obtained.

In the present invention, at least one selected from the groupconsisting of a Pluronic-type nonionic surfactant and a nonionicsurfactant represented by Formula (1) below and/or nonionic surfactantrepresented by Formula (2) below is used. Each of these nonionicsurfactants may be used alone, or two or more kinds of these may be usedin combination.

In Formula (1), R¹ represents a C6-C26 hydrocarbon group, and drepresents an integer.

In Formula (2), R² and R³ are the same or different and each represent aC6-C26 hydrocarbon group, and e represents an integer.

First, the nonionic surfactants represented by Formula (1) and/or byFormula (2) will be described. Among these surfactants, the nonionicsurfactant represented by Formula (1) is preferred because the effectsof the present invention can be more suitably achieved.

R¹ in Formula (1) represents a C6-C26 hydrocarbon group. If R¹ is ahydrocarbon group having 5 or less carbon atoms, such a nonionicsurfactant poorly permeates through rubber and migrates to the rubbersurface too fast, as a result of which the rubber surface tends to havepoor appearance. Also, if R¹ is a hydrocarbon group having 27 or morecarbon atoms, such a material is difficult to obtain or expensive and isthus inappropriate. When R¹ is a hydrocarbon group having a carbonnumber within the above range, blooming of the nonionic surfactant canbe suitably controlled and the effects of the present invention can bemore suitably achieved.

R¹ is preferably a hydrocarbon group having 8 to 24 carbon atoms, morepreferably 10 to 22 carbon atoms, still more preferably 14 to 20 carbonatoms.

Examples of the C6-C26 hydrocarbon group as R¹ include C6-C26 alkenylgroups, C6-C26 alkynyl groups, and C6-C26 alkyl groups.

Examples of the C6-C26 alkenyl groups include 1-hexenyl, 2-hexenyl,1-octenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl,pentadecenyl, heptadecenyl, octadecenyl, icosenyl, tricosenyl, andhexacosenyl groups.

Examples of the C6-C26 alkynyl groups include hexynyl, heptynyl,octynyl, nonynyl, decynyl, undecynyl, dodecynyl, tridecynyl,tetradecynyl, pentadecynyl, heptadecynyl, octadecynyl, icosynyl,tricosynyl, and hexacosynyl groups.

Examples of the C6-C26 alkyl groups include hexyl, heptyl, 2-ethylhexyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,octadecyl, heptadecyl, octadecyl, icosyl, tricosyl, and hexacosylgroups.

R¹ is preferably a C6-C26 alkenyl group or a C6-C26 alkynyl group, morepreferably a C6-C26 alkenyl group.

A nonionic surfactant with a greater d (integer) has a higher value ofHLB, which shows hydrophile-lipophile balance, and tends to migratefaster to the rubber surface. In the present invention, the d value isnot particularly limited, and may be appropriately chosen according tothe service conditions, purpose, or the like. In particular, the d valueis preferably 2 to 25, more preferably 4 to 20, still more preferably 8to 16, particularly preferably 10 to 14.

Examples of the nonionic surfactant represented by Formula (1) includeethylene glycol monooleate, ethylene glycol monopalmeate, ethyleneglycol monopalmitate, ethylene glycol monovaccenate, ethylene glycolmonolinoleate, ethylene glycol monolinolenate, ethylene glycolmonoarachidonate, ethylene glycol monostearate, ethylene glycolmonocetylate, and ethylene glycol monolaurate. Each of these may be usedalone, or two or more of these may be used in combination. In view ofready availability and cost, ethylene glycol monooleate, ethylene glycolmonolaurate, ethylene glycol monostearate, and ethylene glycolmonopalmitate are preferred among these.

R² and R³ in Formula (2) are the same or different and each represent aC6-C26 hydrocarbon group. If R² or R³ is a hydrocarbon group having 5 orless carbon atoms, such a nonionic surfactant poorly permeates throughrubber and migrates to the rubber surface too fast, as a result of whichthe rubber surface tends to have poor appearance. If R² or R³ is ahydrocarbon group having 27 or more carbon atoms, such a material isdifficult to obtain or expensive and is thus inappropriate. When R² andR³ are each a hydrocarbon group having a carbon number within the aboverange, blooming of the nonionic surfactant can be suitably controlledand the effects of the present invention can be more suitably achieved.

R² and R³ are each preferably a hydrocarbon group having 8 to 24 carbonatoms, more preferably 10 to 22 carbon atoms, still more preferably 14to 20 carbon atoms.

Examples of the C6-C26 hydrocarbon group as R² or R³ include C6-C26alkenyl groups, C6-C26 alkynyl groups, and C6-C26 alkyl groups.

Examples of the C6-C26 alkenyl groups, C6-C26 alkynyl groups, and C6-C26alkyl groups include those groups mentioned for R¹ above.

R² and R³ are each preferably a C6-C26 alkenyl group or a C6-C26 alkynylgroup, more preferably a C6-C26 alkenyl group.

A nonionic surfactant with a greater e (integer) has a higher value ofHLB, which shows hydrophile-lipophile balance, and tends to migratefaster to the rubber surface. In the present invention, the e value isnot particularly limited, and may be appropriately chosen according tothe service conditions, purpose, or the like. In particular, the e valueis preferably 2 to 25, more preferably 4 to 20, still more preferably 8to 16, particularly preferably 10 to 14.

Examples of the nonionic surfactant represented by Formula (2) includeethylene glycol dioleate, ethylene glycol dipalmeate, ethylene glycoldipalmitate, ethylene glycol divaccenate, ethylene glycol dilinoleate,ethylene glycol dilinolenate, ethylene glycol diarachidonate, ethyleneglycol distearate, ethylene glycol dicetylate, and ethylene glycoldilaurate. Each of these may be used alone, or two or more of these maybe used in combination. In view of ready availability and cost, ethyleneglycol dioleate, ethylene glycol dilaurate, ethylene glycol distearate,and ethylene glycol dipalmitate are preferred among these.

The Pluronic-type nonionic surfactant will be described below.

The Pluronic-type nonionic surfactant is also called polyoxyethylenepolyoxypropylene glycol, polyoxyethylene polyoxypropylene block polymer,or polypropylene glycol ethylene oxide adduct, and is generally anonionic surfactant represented by Formula (I) below. As shown inFormula (I), the Pluronic-type nonionic surfactant contains on bothsides thereof a hydrophilic group having an ethylene oxide structure,and also contains a hydrophobic group having a propylene oxide structurebetween the hydrophilic groups.

In Formula (I), a, b, and c each represent an integer.

The degree of polymerization of the polypropylene oxide block (b inFormula (I)) and the number of polyethylene oxide units added (a+c inFormula (I)) in the Pluronic-type nonionic surfactant are notparticularly limited, and may be appropriately chosen according to theservice conditions, purpose, or the like. A surfactant with a higherproportion of the polypropylene oxide block tends to have higheraffinity for rubber and thus to migrate to the rubber surface at aslower rate. In particular, in order to suitably control blooming of thenonionic surfactant and more suitably achieve the effects of the presentinvention, the degree of polymerization of the polypropylene oxide block(b in Formula (I)) is preferably 100 or less, more preferably 10 to 70,still more preferably 10 to 60, particularly preferably 20 to 60, mostpreferably 20 to 45. For the same reason, the number of polyethyleneoxide units added (a+c in Formula (I)) is preferably 100 or less, morepreferably 3 to 65, still more preferably 5 to 55, particularlypreferably 5 to 40, most preferably 10 to 40. When the degree ofpolymerization of the polypropylene oxide block and the number ofpolyethylene oxide units added are within the respective rangesdescribed above, blooming of the nonionic surfactant can be suitablycontrolled and the effects of the present invention can be more suitablyachieved.

Examples of the Pluronic-type nonionic surfactant include Pluronicseries available from BASF Japan Ltd., Newpol PE series available fromSanyo Chemical Industries, Ltd., Adeka Pluronic L or F series availablefrom Adeka Corporation, Epan series available from DKS Co. Ltd., andPronon series or UNILUB available from NOF corporation. Each of thesemay be used alone, or two or more of these may be used in combination.

The combined amount of the nonionic surfactant represented by Formula(1), the nonionic surfactant represented by Formula (2), and thePluronic-type nonionic surfactant (the amount of the nonionicsurfactants), per 100 parts by mass of the rubber component is 0.1 partsby mass or more, preferably 0.3 parts by mass or more, more preferably0.5 parts by mass or more, still more preferably 1 part by mass or more,particularly preferably 1.2 parts by mass or more. If the combinedamount is less than 0.1 parts by mass, the effects of the presentinvention cannot be sufficiently obtained. Also, the combined amount is5.0 parts by mass or less, preferably 4.0 parts by mass or less, morepreferably 3.0 parts by mass or less, still more preferably 2.0 parts bymass or less. If the combined amount is more than 5.0 parts by mass,elongation at break and fuel economy are deteriorated.

A phenylenediamine antioxidant is used in the present invention. The useof such a specific antioxidant together with the specific surfactant(s)and optionally a petroleum-derived wax can provide excellent ozoneresistance over a wide temperature range and also sufficiently suppressdiscoloration. The antioxidant may be used alone or in combinations oftwo or more.

Examples of the phenylenediamine antioxidant includeN-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine,N-(1,4-dimethylpentyl)-N′-phenyl-p-phenylenediamine,N-isopropyl-N′-phenyl-p-phenylenediamine,N,N′-diphenyl-p-phenylenediamine, N,N′-di-2-naphthyl-p-phenylenediamine,N-cyclohexyl-N′-phenyl-p-phenylenediamine,N,N′-bis(1-methylheptyl)-p-phenylenediamine,N,N′-bis(1,4-dimethylpentyl)-p-phenylenediamine,N,N′-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine,N-4-methyl-2-pentyl-N′-phenyl-p-phenylenediamine,N,N′-diaryl-p-phenylenediamine, hindered diaryl-p-phenylenediamine,phenylhexyl-p-phenylenediamine, and phenyloctyl-p-phenylenediamine.Preferred among these isN-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine.

A quinone antioxidant may be used together with the phenylenediamineantioxidant. Examples of the quinone antioxidant include benzoquinoneantioxidants, hydroquinone antioxidants, catechol antioxidants,quinonediimine antioxidants, quinomethane antioxidants, andquinodimethane antioxidants. Preferred among these are quinonediimineantioxidants.

Examples of the quinonediimine antioxidants includeN-isopropyl-N′-phenyl-p-quinonediimine,N-(1,3-dimethylbutyl)-N′-phenylquinonediimine,N,N′-diphenyl-p-quinonediimine, N-cyclohexyl-N′-phenyl-p-quinonediimine,N-n-hexyl-N′-phenyl-p-quinonediimine, and N,N′-dioctyl-p-quinonediimine.Preferred among these is N-(1,3-dimethylbutyl)-N′-phenylquinonediimine(6QDI).

The amount of phenylenediamine antioxidant per 100 parts by mass of therubber component is 1.0 part by mass or more, preferably 1.5 parts bymass or more, more preferably 1.8 parts by mass or more. If the amountis less than 1.0 part by mass, sufficient ozone resistance or elongationat break is not obtained. Also, the amount is 8.0 parts by mass or less,preferably 5.0 parts by mass or less, more preferably 4.5 parts by massor less. If the amount is more than 8.0 parts by mass, fuel economy isdeteriorated and discoloration (brown discoloration) occurs.

In the rubber composition of the present invention, the combined amountof carbon black and silica per 100 parts by mass of the rubber componentis 20 to 45 parts by mass, preferably 24 to 43 parts by mass, morepreferably 27 to 40 parts by mass. If the combined amount is less than20 parts by mass, elongation at break, ozone resistance, ordiscoloration resistance is reduced, while if the combined amount ismore than 45 parts by mass, fuel economy or ozone resistance,particularly fuel economy, is deteriorated. Moreover, if an increasedamount of oil is used to ensure better crack growth resistance, theantioxidant and wax bloom faster so that long-term ozone resistancetends to be reduced. Moreover, when the combined amount of carbon blackand silica is within the above range, elongation at break is markedlyreduced by the addition of the aforementioned surfactant. In contrast,in the present invention using a high viscosity polybutadiene rubbersuch as the specific non-oil extended polybutadiene rubber or thespecific oil extended polybutadiene rubber together with the specificsurfactant, both discoloration resistance and ozone resistance can beensured while maintaining or improving good elongation at break and goodfuel economy.

In the present invention, either carbon black or silica may be usedalone, or both may be used in combination.

The rubber composition of the present invention preferably containscarbon black. The addition of carbon black produces a reinforcing effectand an UV-blocking effect and therefore the effects of the presentinvention can be well achieved. Examples of usable carbon black includeGPF, FEF, HAF, ISAF, and SAF.

The carbon black preferably has a nitrogen adsorption specific surfacearea (N₂SA) of 20 to 200 m²/g, more preferably 30 to 60 m²/g. If theN₂SA is less than 20 m²/g, durability or handling stability may bereduced. If the N₂SA is more than 200 m²/g, sufficient fuel economy orprocessability may not be obtained. Herein, the nitrogen adsorptionspecific surface area of carbon black can be determined in conformitywith JIS K 6217-2:2001.

In the case of the rubber composition containing carbon black, theamount of carbon black per 100 parts by mass of the rubber component ispreferably 2 to 45 parts by mass, more preferably 20 to 43 parts bymass, still more preferably 24 to 40 parts by mass. If the amount isless than 2 parts by mass, sufficient reinforcing properties tend not tobe obtained and durability, elongation at break, handling stability,ozone resistance, or discoloration resistance tends to be deteriorated.If the amount is more than 45 parts by mass, fuel economy or ozoneresistance may be deteriorated.

The rubber composition of the present invention may contain silica. Anysilica may be used, and examples include dry silica (anhydrous silica)and wet silica (hydrous silica). Wet silica (hydrous silica) ispreferred because it has many silanol groups.

In the case of the rubber composition containing silica, the amount ofsilica may be appropriately chosen in view of the effects of the presentinvention and the like. For example, for use in sidewalls or clinches,the amount of silica is preferably 0.1 to 40 parts by mass per 100 partsby mass of the rubber component. If the amount is more than 40 parts bymass, fuel economy may be deteriorated.

In the case of the rubber composition containing silica, the rubbercomposition preferably contains a silane coupling agent together withsilica. However, if the amount of carbon black is 15 parts by mass ormore and the amount of silica is 12 parts by mass or less, it isunnecessary to add a silane coupling agent because in this case thesilica is immobilized in the carbon gel and is thus unlikely toreaggregate.

Any silane coupling agent conventionally used in combination with silicain the rubber industry may be used. Examples include sulfide silanecoupling agents such as bis(3-triethoxysilylpropyl)disulfide; mercaptosilane coupling agents such as 3-mercaptopropyltrimethoxysilane; vinylsilane coupling agents such as vinyltriethoxysilane; amino silanecoupling agents such as 3-aminopropyltriethoxysilane; glycidoxy silanecoupling agents such as γ-glycidoxypropyltriethoxysilane; nitro silanecoupling agents such as 3-nitropropyltrimethoxysilane; and chloro silanecoupling agents such as 3-chloropropyltrimethoxysilane.

The rubber composition of the present invention may contain a naturallyoccurring wax containing 40 to 98% by mass of an ester component. Thenaturally occurring wax captures the antioxidant to inhibit theantioxidant from migrating to the tire surface. Accordingly, blooming ofthe antioxidant moderately slows down so that the antioxidant remains inthe rubber composition for a long period of time. Thus, excellent ozoneresistance can be ensured over a wide temperature range. Moreover, theantioxidant can be prevented from excessively blooming in a short periodof time and therefore white discoloration and brown discoloration of thetire surface can also be prevented.

The amount (content) of the ester component based on 100% by mass of thenaturally occurring wax is preferably 50 to 98% by mass, more preferably60 to 98% by mass. A wax containing more than 98% by mass of the estercomponent tends to have reduced flexibility and thus to form a morefragile thin film.

The amount of free alcohol or free fatty acid based on 100% by mass ofthe naturally occurring wax is independently preferably 10% by mass orless, more preferably 7% by mass or less. If the amount is more than 10%by mass, ozone resistance, particularly at low temperatures, tends to bedeteriorated.

Regarding the carbon number distribution (molecular weightdistribution), i.e., the softening point distribution, of the naturallyoccurring wax, the naturally occurring wax preferably contains acomponent having a softening point of 40° C. to 95° C., more preferably60° C. to 90° C., still more preferably 70° C. to 86° C. In such case,both discoloration resistance and ozone resistance can be ensured.

The softening point distribution of the wax can be determined, forexample, by gas chromatography (GC) or by measuring heat flow (mW/g) ata rate of temperature rise of 5° C./min from −30° C. to 100° C. using adifferential scanning calorimeter (DSC). When DSC is used, the presenceof a component having a predetermined softening point in the wax can bedetermined based on whether or not the curve of heat flow versustemperature at the predetermined temperature point is below the baselinetoward the endothermic side.

The naturally occurring wax (natural wax) may be a wax obtained byremoving free fatty acids, free alcohols, resins, or the like from anaturally occurring wax. Suitable examples include refined naturallyoccurring waxes (refined natural waxes) such as refined gramineous plantwaxes extracted from gramineous plants, e.g., refined rice wax, refinedcandelilla wax, refined beeswax, refined sugar cane wax, and the like.The use of such a refined naturally occurring wax, which containsreduced amounts of polar components, such as free fatty acids, freealcohols or resins, and has an increased relative proportion ofhydrocarbons, not only improves the compatibility with low polarityrubber and the uniformity of the resulting film but also suppressesblooming. Therefore, brown discoloration of the rubber surface can beprevented.

The refined naturally occurring wax may be, for example, a wax obtainedby removing at least one selected from the group consisting of freefatty acids, free alcohols, and resins from a naturally occurring wax.The naturally occurring wax may be any wax other than petroleum-derivedwaxes. Examples include plant waxes such as waxes extracted fromgramineous plants, e.g., rice wax, candelilla wax, carnauba wax, Japanwax, and jojoba wax; animal waxes such as beeswax, lanolin, andspermaceti; mineral waxes such as ozokerite, ceresin, and petrolatum;hydrogenated natural fats and oils such as hydrogenated castor oil,hydrogenated soybean oil, hydrogenated rapeseed oil, and hydrogenatedbeef tallow; and refined products of the foregoing. Moreover, thenaturally occurring wax may be derived from a genetically modified plantor animal. The removal may be carried out by any method that can removefree alcohols, free fatty acids, or resins, and known methods may beused.

The naturally occurring wax is preferably a refined plant wax, morepreferably a refined wax extracted from a gramineous plant, particularlypreferably refined rice wax. Moreover, the use of refined rice wax incombination with a petroleum-derived wax can suppress formation ofcracks and provide excellent ozone resistance over a wide temperaturerange from low to high temperatures, and at the same time cansufficiently prevent brown discoloration and white discoloration.Furthermore, although the film may be broken by dynamic stimuli imposedduring service of the tire, refined rice wax, which is a fatty acidester and can be readily mixed with petroleum-derived waxes,antioxidants, and surfactants, can be expected to prevent crackformation and discoloration for a long period of time. The naturallyoccurring wax may be used alone or in combinations of two or more.

The amount of the naturally occurring wax per 100 parts by mass of therubber component is preferably 0.01 parts by mass or more, morepreferably 0.05 parts by mass or more, still more preferably 0.10 partsby mass or more. If the amount is less than 0.01 parts by mass, aparticular effect such as improved ozone resistance may not be found.Also, the amount is preferably 5.0 parts by mass or less, morepreferably 2.5 parts by mass or less, still more preferably 1.5 parts bymass or less, particularly preferably 0.5 parts by mass or less. If theamount is more than 5.0 parts by mass, the amount of blooms of theantioxidant decreases and thus ozone resistance and elongation at breakare reduced.

A petroleum-derived wax is preferably added in the present invention.The petroleum-derived wax may be any of waxes derived from petroleumresources, including, for example, paraffin wax and microcrystallinewax. In particular, for excellent ozone resistance over a widetemperature range, the petroleum-derived wax preferably contains C20 toC32 normal alkanes. The petroleum-derived wax may be used alone or incombinations of two or more.

The petroleum-derived wax containing C20 to C32 normal alkanes is notparticularly limited, and may be, for example, a petroleum-derived waxcontaining a predetermined amount of C20 to C55 normal alkanes. Inparticular, for excellent ozone resistance, the petroleum-derived waxmay suitably be a wax having a normal alkane content of 70% by mass ormore, more suitably 80% by mass or more.

The combined amount of C20 to C32 normal alkanes based on 100% by massof the petroleum-derived wax is preferably 25% by mass or more, morepreferably 35% by mass or more. If the combined amount is less than 25%by mass, sufficient ozone resistance may not be obtained at atemperature range of 20° C. or lower. The combined amount is preferably90% by mass or less, more preferably 50% by mass or less. If thecombined amount is more than 90% by mass, discoloration resistance maybe reduced.

The combined amount of C33 to C44 normal alkanes based on 100% by massof the petroleum-derived wax is preferably 25% by mass or more, morepreferably 35% by mass or more. If the combined amount is less than 25%by mass, sufficient ozone resistance may not be obtained at atemperature range from about 40° C. to about 50° C. The combined amountis preferably 90% by mass or less, more preferably 50% by mass or less.If the combined amount is more than 90% by mass, a large amount of C33to C44 normal alkanes tend to bloom at a temperature range from about40° C. to about 50° C., causing white discoloration.

The combined amount of C45 to C47 normal alkanes based on 100% by massof the petroleum-derived wax is preferably 0.5% by mass or more, morepreferably 2% by mass or more. If the combined amount is less than 0.5%by mass, crack resistance may be slightly deteriorated at a temperaturerange of about 60° C. The combined amount is preferably 10% by mass orless, more preferably 5% by mass or less. If the combined amount is morethan 10% by mass, resistance to discoloration (white discoloration)tends to be deteriorated at a temperature range of about 60° C.

The combined amount of C48 and higher normal alkanes based on 100% bymass of the petroleum-derived wax is preferably 10% by mass or less,more preferably 5% by mass or less. In such case, good resistance todiscoloration (white discoloration) can be achieved at a temperaturerange of 60° C. or higher.

In the case of the rubber composition containing a petroleum-derivedwax, the amount of petroleum-derived wax per 100 parts by mass of therubber component is preferably 0.3 parts by mass or more, morepreferably 1.0 part by mass or more. If the amount is less than 0.3parts by mass, a particular effect such as improved ozone resistance maynot be found. Also, the amount is preferably 6.0 parts by mass or less,more preferably 4.0 parts by mass or less, still more preferably 2.0parts by mass or less. If the amount is more than 6.0 parts by mass, toolarge an amount of blooms is formed, which may cause white discolorationof the tire and reduce elongation at break or fuel economy.

In the present invention, the blending ratio of the naturally occurringwax and the petroleum-derived wax [(mass of naturally occurringwax)/(mass of petroleum-derived wax)] is preferably 2/98 to 70/30, morepreferably 5/95 to 50/50, still more preferably 10/90 to 40/60. In suchcase, both ozone resistance and discoloration resistance can be moresuitably ensured.

The rubber composition of the present invention may contain a softener.When a softener is added, blooming of the above-mentioned antioxidant,naturally occurring wax, nonionic surfactant, and petroleum-derived waxcan be suitably controlled and the effects of the present invention canbe better achieved.

Examples of the softener include oil, and resins such as C5 petroleumresin, C9 petroleum resin, coumarone indene resin, indene resin,non-reactive alkylphenol resins, or aromatic vinyl polymers obtained bypolymerizing α-methylstyrene and/or styrene. The softener may beappropriately selected depending on the migration rate of the wax orantioxidant, and the like. Among these, oil is preferred because theeffects of the present invention can be suitably achieved.

Examples of the oil include process oils, vegetable fats and oils, andmixtures of these. Examples of the process oils include paraffinicprocess oils, aromatic process oils, and naphthenic process oils.Specific examples of the paraffinic process oil include PW-32, PW-90,PW-150, and PS-32 available from Idemitsu Kosan Co., Ltd. Specificexamples of the aromatic process oil include AC-12, AC-460, AH-16,AH-24, and AH-58 available from Idemitsu Kosan Co., Ltd. Examples of thevegetable oils and fats include castor oil, cottonseed oil, linseed oil,rapeseed oil, soybean oil, palm oil, coconut oil, peanut oil, rosin,pine oil, pine tar, tall oil, corn oil, rice bran oil, safflower oil,sesame oil, olive oil, sunflower oil, palm kernel oil, camellia oil,jojoba oil, macadamia nut oil, and tung oil. Each of these may be usedalone, or two or more of these may be used in combination. Among these,aromatic process oils are preferred because the effects of the presentinvention can be suitably achieved.

In the case of the rubber composition containing a softener, the amountof softener per 100 parts by mass of the rubber component is preferably1.0 part by mass or more, more preferably 3.0 parts by mass or more.Also, the amount of softener is preferably 40 parts by mass or less,more preferably 30 parts by mass or less, still more preferably 14 partsby mass or less, particularly preferably 8.0 parts by mass or less. Ifthe amount of softener, which itself blooms to the tire surface, isadjusted to the above range, blooming of the above-mentionedantioxidant, naturally occurring wax, nonionic surfactant, andpetroleum-derived wax can be suitably controlled and the effects of thepresent invention can be more suitably achieved.

The rubber composition of the present invention may appropriatelycontain compounding agents commonly used in the manufacture of rubbercompositions, such as stearic acid, zinc oxide, a vulcanizing agent, ora vulcanization accelerator, in addition to the above-mentionedcomponents.

Sulfur is preferably used as a vulcanizing agent in the presentinvention. In this case, a moderate amount of crosslinks are formedbetween polymers, as a result of which blooming of the above-mentionedantioxidant, naturally occurring wax, nonionic surfactant, andpetroleum-derived wax can be suitably controlled and the effects of thepresent invention can be more suitably achieved. Examples of the sulfurinclude those commonly used in the rubber industry, such as powderedsulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highlydispersible sulfur, and soluble sulfur. Each of these may be used alone,or two or more of these may be used in combination.

The amount of sulfur per 100 parts by mass of the rubber component ispreferably 0.1 parts by mass or more, more preferably 0.5 parts by massor more, still more preferably 1.0 part by mass or more. If the amountis less than 0.1 parts by mass, insufficient hardness (Hs) aftervulcanization may be obtained and co-curing with neighboring rubbercompounds may be insufficient. The amount of sulfur is preferably 6.0parts by mass or less, more preferably 5.0 parts by mass or less, stillmore preferably 4.0 parts by mass or less, particularly preferably 3.0parts by mass or less. If the amount is more than 6.0 parts by mass,crack growth resistance, ozone resistance, elongation at break, ordurability may be deteriorated.

Besides sulfur, an alkylphenol-sulfur chloride condensate (for example,Tackirol V200 available from Taoka Chemical Co., Ltd.) may be used as avulcanizing agent in the present invention.

The rubber composition of the present invention may be prepared by knownmethods, such as, for example, by kneading the aforementioned componentsusing a rubber kneading machine such as an open roll mill or a Banburymixer, and then vulcanizing the mixture.

The rubber composition of the present invention can be used in any tirecomponent and can be suitably used as a rubber composition for tireouter layers that forms a surface (outer face) of a tire, such as atread, a wing, a sidewall, or a clinch, and more suitably a sidewall ora wing.

A wing refers to a component positioned between a tread and a sidewallin the shoulder area. Specifically, it is a component shown in FIGS. 1and 3 of JP 2007-176267 A, or the like.

A clinch refers to a rubber part which is located at a lower portion ofa sidewall and covers the area contacting a rim, and is also called aclinch apex or a rubber chafer. Specifically, it is a component shownin, for example, FIG. 1 of JP 2008-75066 A, or the like.

The pneumatic tire of the present invention can be manufactured usingthe rubber composition by usual methods. Specifically, the rubbercomposition, before vulcanization, is extruded and processed into theshape of a tire component such as a tread, a wing, a sidewall, or aclinch, and then assembled with other tire components in a conventionalmanner on a tire building machine to build an unvulcanized tire, whichis then heated and pressurized in a vulcanizer to form a tire.

EXAMPLES

The present invention is specifically described with reference to, butnot limited to, examples.

<Preparation of Terminal Modifier>

An amount of 20.8 g of 3-(N,N-dimethylamino)-propyltrimethoxysilane(AZmax. Co) was put in a 250-mL graduated flask in a nitrogenatmosphere, and then anhydrous hexane (Kanto Chemical Co., Inc.) wasadded to a total volume of 250 mL.

Preparation Example

To a sufficiently nitrogen-purged 30-L pressure-proof vessel were added18 L of cyclohexane (Kanto Chemical Co., Inc.), 2000 g of butadiene(Takachiho Chemical Industrial Co., Ltd.), and 53 mmol of diethyl ether(Kanto Chemical Co., Inc.), followed by heating to 60° C. Next, 16.6 mLof butyllithium (Kanto Chemical Co., Inc.) was added and stirred for 3hours. Subsequently, 12 mL of a 0.4 mol/L solution of silicontetrachloride in hexane was added and stirred for 30 minutes.Thereafter, 13 mL of the terminal modifier was added and stirred for 30minutes. To the reaction solution was added 2 mL of a solution of 0.2 gof 2,6-tert-butyl-p-cresol (Ouchi Shinko Chemical Industrial Co., Ltd.)in methanol (Kanto Chemical Co., Inc.). The resulting reaction solutionwas put in a stainless steel vessel containing 18 L of methanol tocollect an aggregate. The aggregate was dried for 24 hours under reducedpressure to give a modified BR.

The chemicals used in the examples and comparative examples arecollectively listed below.

NR: TSR20

IR: IR2200

BR 1: BR1250H (tin-modified BR polymerized using a lithium initiator,tin atom content: 250 ppm) available from Zeon Corporation

BR 2: VCR617 (SPB-containing BR, SPB content: 17% by mass, melting pointof SPB: 200° C.) available from Ube Industries, Ltd.

BR 3: VCR412 (SPB-containing BR, SPB content: 12% by mass, melting pointof SPB: 200° C.) available from Ube Industries, Ltd.

BR 4: BUNA-CB22 (rare earth-catalyzed BR synthesized using a Ndcatalyst) available from LANXESS

BR 5: BUNA-CB24 (rare earth-catalyzed BR synthesized using a Ndcatalyst) available from LANXESS

BR 6: BR730 (rare earth-catalyzed BR synthesized using a Nd catalyst)available from JSR Corporation

BR 7: BR150B (BR synthesized using a Co catalyst) available from UbeIndustries, Ltd.

BR 8: modified BR prepared in the above Preparation Example (modified BRsynthesized using a lithium catalyst)

BR 9: BUNA-CB29 TDAE (rare earth-catalyzed BR synthesized using a Ndcatalyst, containing 37.5 parts by mass of TDAE per 100 parts by mass ofthe rubber component) available from LANXESS

BR10: BUNA-CB29 MES (rare earth-catalyzed BR synthesized using a Ndcatalyst, containing 37.5 parts by mass of MES per 100 parts by mass ofthe rubber component) available from LANXESS

SBR: SBR1502 available from Zeon Corporation

Carbon black 1 (N550): Shoblack N550 (N₂SA: 42 m²/g, DBP oil absorption:115 mL/100 g) available from Cabot Japan K. K.

Carbon black 2 (N220): Shoblack N220 (N₂SA: 111 m²/g, DBP oilabsorption: 115 mL/100 g) available from Cabot Japan K. K.

Silica: Ultrasil VN3 available from Evonik Degussa

Oil: Vivatec 500 (TDAE, low polycyclic aroma oil) available from H&R

Petroleum C5 resin: Marukarez T-100AS (softening point: 102° C.)available from Maruzen Petrochemical Co., Ltd.

Stearic acid: Stearic acid “Tsubaki” available from NOF Corporation

Zinc oxide: Ginrei R available from Toho Zinc Co., Ltd.

Sulfur: SEIMI sulfur OT (insoluble sulfur, oil content: 10%) availablefrom Nippon Kanryu Industry Co., Ltd.

Vulcanization accelerator TBBS: Nocceler NS(N-tert-butyl-2-benzothiazolylsulfenamide) available from Ouchi ShinkoChemical Industrial Co., Ltd.

Petroleum-derived wax: Trial product (normal alkane content: 85% by masson average)

Naturally occurring wax: Refined rice wax S-100 (softening pointdistribution: 77° C. to 83° C., ester component: 95% by mass, free fattyacid: 4% by mass, free alcohol: 1% by mass, hydrocarbon: 1% by mass)available from Yokozeki Oil & Fat Industries Co., Ltd.

<Surfactant 1>: Ionet DO600 (the principal ingredient is a compoundrepresented by the formula below which corresponds to Formula (2) inwhich R² and R³ are each —C₁₇H₃₃ and e is 12) available from SanyoChemical Industries, Ltd.C₁₇H₃₃COO(CH₂CH₂O)₁₂COC₁₇H₃₃

<Surfactant 2>: Ionet PO600 (the principal ingredient is a compoundrepresented by the formula below which corresponds to Formula (1) inwhich R¹ is —C₁₇H₃₃ and d is 12) available from Sanyo ChemicalIndustries, Ltd.C₁₇H₃₃COO(CH₂CH₂O)₁₂H

<Surfactant 3>: NEWPOL PE-64 (Pluronic-type nonionic surfactant,copolymer of PEG/PPG (25/30), Formula (I) in which a+c is 25 and b is30) available from Sanyo Chemical Industries, Ltd.

<Surfactant 4>: NEWPOL PE-74 (Pluronic-type nonionic surfactant,copolymer of PEG/PPG (30/35), Formula (I) in which a+c is 30 and b is35) available from Sanyo Chemical Industries, Ltd.

<Surfactant 5>: Polyoxyethylene sorbitan monostearate available fromKanto Chemical Co., Inc.

<Surfactant 6>: Polyoxyethylene sorbitan trioleate available from KantoChemical Co., Inc.

<Surfactant 7>: Polyoxyethylene dodecyl ether available from KantoChemical Co., Inc.

<Surfactant 8>: Ethylene glycol dibutyl ether available from TokyoChemical Industry Co., Ltd.

Antioxidant 6C: Antigene 6C(N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD)) availablefrom Sumitomo Chemical Co., Ltd.

Antioxidant TMQ: Nocrac 224 (2,2,4-trimethyl-1,2-dihydroquinolinepolymer) available from Ouchi Shinko Chemical Industrial Co., Ltd.

The properties of BR 1 to BR 10 are summarized in Table 1. BR 2, BR 4,and BR 6 correspond to the non-oil extended polybutadiene rubber, and BR9 and BR 10 correspond to the oil extended polybutadiene rubber.

TABLE 1 Microstructure GPC analysis SPB (% by mass) Oil extended/ Mwcontent ML₁₊₄ Cis Vinyl Trade name, etc. Catalyst Non-oil extended Mw/MnMn (×10⁴) (×10⁴) (% by mass) 100° C. content content BR 1 BR1250HLithium catalyst Non-oil extended BR 1.39 33 46 — 51 40 10.0 BR 2 VCR617Cobalt catalyst Non-oil extended BR 2.5 17 44 17 62 96 2.2 BR 3 VCR412Cobalt catalyst Non-oil extended BR 2.6 17 43 12 45 96 2.2 BR 4 CB22Neodymium catalyst Non-oil extended BR 1.6 37 59 — 63 97 0.6 BR 5 CB24Neodymium catalyst Non-oil extended BR 1.78 28 50 — 44 97 0.7 BR 6 BR730Neodymium catalyst Non-oil extended BR 1.88 31 58 — 51 97 0.9 BR 7BR150B Cobalt catalyst Non-oil extended BR 2.3 19 44 — 43 96 2.1 BR 8Modified BR Lithium catalyst Non-oil extended BR 1.19 46 55 — 60 38 13synthesized in Preparation Example BR 9 CB29 TDAE, Neodymium catalystOil extended BR 2.33 32.6 76 — 37 97 0.6 oil extended BR 10 CB29 MES,Neodymium catalyst Oil extended BR 3.86 19.1 73.7 — 37 97 0.6 oilextended

The carbon number distribution of the petroleum-derived wax wasdetermined by the method below. Table 4 shows the results.

The carbon number distribution was measured using a capillary GCanalyzer and a capillary column coated with aluminum with helium carriergas at a flow rate of 4 mL/min, a column temperature of 180° C. to 390°C., and a rate of temperature rise of 15° C./min.

Examples and Comparative Examples

The chemicals in formulation amounts shown in Table 2 or 3, except thesulfur and vulcanization accelerator, were kneaded in a 1.7-L Banburymixer available from Kobe Steel, Ltd. Then, the sulfur and vulcanizationaccelerator were added to the kneaded mixture and they were kneadedusing an open roll mill to prepare an unvulcanized rubber composition.The unvulcanized rubber composition was formed into the shapes of atread, a wing, a sidewall, and a clinch, and they were assembled withother tire components to build an unvulcanized tire, which was thenvulcanized at 170° C. to prepare a test tire (205/65R15). The test tiresthus prepared were evaluated for performance by the following tests.

<Hardness Measurement>

Rubber samples were cut out of the sidewalls of the tires. Next, thehardness of the rubber samples was measured at 25° C. using a durometerin conformity with JIS K 6253 (Shore A measurement). The compositionswere adjusted to have a hardness of 54±1 in order to permit comparisonof the elongation at break (EB) of the compositions.

<Elongation at Break>

Rubber samples were cut out of the sidewalls of the tires. Next,specimens were prepared from the rubber samples using a No. 3 dumbbelldie, and then subjected to a tensile test at room temperature inconformity with JIS K 6251 “Rubber, vulcanized orthermoplastics—Determination of tensile stress-strain properties” tomeasure the elongation at break EB (%). The EB values were used tocalculate an index [(EB of each formulation)/(EB of Comparative Example1)×100], where the EB value of Comparative Example 1 was taken as 100. Ahigher index indicates better elongation at break. The target EB indexwas 95 or higher.

<Fuel Economy>

Rubber samples were cut out of the sidewalls of the tires. Then, the tanδ of the rubber samples of the formulations was measured using aviscoelastic spectrometer VES (Iwamoto Seisakusho) at a temperature of70° C., an initial strain of 10%, and a dynamic strain of ±1%. Using theequation below, the tan δ values were expressed as an index relative tothe tan δ of Comparative Example 1 (=100). A higher index indicatesbetter rolling resistance properties (fuel economy). The target RR indexwas 90 or higher.(Rolling resistance index)=(tan δ of Comparative Example 1)/(tan δ ofeach formulation)×100<Ozone Cracking Resistance Test>

Road tests were conducted in the United Arab Emirates in the Middle East(a hot climate) for approximately a year (including summer), and inHokkaido, Japan (a cold climate) for approximately a year (includingwinter). The degree of cracking in the test was evaluated based on thecriteria below. A greater number indicates better ozone resistance(crack resistance). The target number was 3+ or higher.

(Criteria)

-   1: A crack or break of 3 mm or more was observed.-   2: A deep crack of at least 1 mm but less than 3 mm was observed.-   3: A deep and relatively large crack of less than 1 mm was observed.-   4: A crack or break was barely visible to the naked eye.-   5: A crack or break was not visible to the naked eye, but visible    with a magnifier (×10).    <Discoloration Test>    (1) Outdoor: Evaluation of Brown Discoloration

The tires were left outside in the sun for 6 months (from winter tosummer) at Kobe city. Then, a* and b* were measured using a colorimeter.Based on the values, the evaluation was made on a five-point scaleaccording to the criteria below. A greater number indicates a lowerdegree of brown discoloration. The target number was 3+ or higher.

(Criteria)

-   1: −(a*+b*)×10≤−30-   2: −30<−(a*+b*)×10≤−20-   3: −20<−(a*+b*)×10≤−10-   4: −10<−(a*+b*)×10≤0-   5: −(a*+b*)×10>0    (2) Indoor: Evaluation of White Discoloration

The tires were left in an indoor warehouse for 6 months (from winter tosummer) at Kobe city. Then, L* was measured using a colorimeter. Basedon the values, the evaluation was made on a five-point scale accordingto the criteria below. A greater number indicates a lower degree ofwhite discoloration. The target number was 3+ or higher.

(Criteria)

-   1: 100−L*≤60-   2: 60<100−L*≤65-   3: 65<100−L*≤70-   4: 70<100−L*≤75-   5: 100−L*>75

TABLE 2 Example 1 2 3 4 5 6 7 8 9 10 11 12 Formulation NR 50   50   50  50   50   50   50   50   50   50   50   50   (parts by IR — — — — — — —— — — — — mass) BR 1 25   25   25   25   25   25   25   25   25   25  25   25   BR 2 25   25   25   25   25   25   25   25   25   25   25  25   BR 3 — — — — — — — — — — — — BR 4 — — — — — — — — — — — — BR 5 — —— — — — — — — — — — BR 6 — — — — — — — — — — — — BR 7 — — — — — — — — —— — — BR 8 — — — — — — — — — — — — BR 9 — — — — — — — — — — — — BR 10 —— — — — — — — — — — — SBR — — — — — — — — — — — — Carbon black 1 (N550)37   37   37   37   37   37   37   37   37   37   37   37   Carbon black2 (N220) — — — — — — — — — — Silica — — — — — — — — — — Oil 7.0 5.5 3.55.5 5.5 5.5 5.5 5.5 6.0 3.0 5.5 5.0 Petroleum C5 resin — — — — — — — — —— Petroleum-derived wax 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 — 5.5 1.3 1.3Naturally occurring wax — — — — — — — —  0.20 1.0 Surfactant 1 0.1 1.54.5 — — — 1.5 1.5 1.5 1.5 1.5 1.5 Surfactant 2 — 1.5 — — — — — — — —Surfactant 3 — — 1.5 — — — — — — — Surfactant 4 — — — 1.5 — — — — — —Surfactant 5 — — — — — — — — — — Surfactant 6 — — — — — — — — — —Surfactant 7 — — — — — — — — — — Surfactant 8 — — — — — — — — — —Antioxidant 6C 3.0 3.0 3.0 3.0 3.0 3.0 1.0 7.0 3.0 3.0 3.5 4.0Antioxidant TMQ 1.0 1.0 1.0 1.0 1.0 1.0 — 1.0 1.0 1.0 1.0 1.0 Stearicacid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulfur 2.0 2.0 2.02.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 5.0 Vulcanization  0.75  0.75  0.75 0.75  0.75  0.75  0.75  0.75  0.75  0.75  0.75  0.75 accelerator TBBSZinc oxide 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 EvaluationHardness — — — — — — — — — — — — result (Hs, adjusted to 54 ± 1) EBindex (Target ≥95) 101    100    95   101    100    100    97   104   105    95   100    96   RR index = tan δ 100    100    92   99   99  99   103    96   102    92   99   98   (Target ≥90) Ozone resistance 3+ 4   4   4   4   4   3+  5   3+  5   4+  4+  in hot climate (on a 5-pointscale, Target ≥3+) Ozone resistance 3+  4   4   4   4   4   3+  5   3+ 5   4   4   in cold climate (on a 5-point scale, Target ≥3+) Browndiscoloration 4   4+  5   4+  4+ 4+  5   4+  5   3+  4   4+  at Kobecity (on a 5-point scale, Target ≥3+) White discoloration 3+  4+  5  4+  4+ 4+  5   4+  5   3+  4   5   at Kobe city (on a 5-point scale,Target ≥3+) Example 13 14 15 16 17 18 19 20 21 22 23 24 Formulation NR50   40   35   40   50   30   — 50   50   50   50   50   (parts by IR —10   — 10   — 20   35   — — — — — mass) BR 1 25   — — 25   25   35   —25   25   25   — 25   BR 2 25   — — — 25   — — — — 17   10   — BR 3 — —— — — — — — — — — — BR 4 — 50   — 25   — 15   65   25   — — — — BR 5 — —— — — — — — — — — — BR 6 — — 65   — — — — — — — — — BR 7 — — — — — — — —— — — — BR 8 — — — — — — — — — — — — BR 9 — — — — — — — — 34.4  11  55   — BR 10 — — — — — — — — — — — 34.4  SBR — — — — — — — — — — — —Carbon black 1 (N550) 37   30   32   34   34   37   10   37   37   37  37   37   Carbon black 2 (N220) — — — — — — 9   — — — — — Silica — — — —5   — 5   — — — — — Oil 4.0 — — 6.0 6.0 5.5 — 8.0 — 5.0 — — Petroleum C5resin — 5.0 5.0 — — — 3.0 — — — — — Petroleum-derived wax 1.3 1.5 1.51.5 1.3 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Naturally occurring 2.0 — — — — — —— — — — — wax Surfactant 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 Surfactant 2 — — — — — — — — — — — — Surfactant 3 — — — — — — — — —— — — Surfactant 4 — — — — — — — — — — — — Surfactant 5 — — — — — — — —— — — — Surfactant 6 — — — — — — — — — — — — Surfactant 7 — — — — — — —— — — — — Surfactant 8 — — — — — — — — — — — — Antioxidant 6C 4.0 4.04.0 3.0 3.0 3.0 4.0 3.0 3.0 3.0 3.0 3.0 Antioxidant TMQ 1.0 1.0 1.0 1.01.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 Sulfur 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.02.0 Vulcanization  0.75  0.75  0.75  0.75  0.75  0.75  1.00  0.75  0.75 0.75  0.75  0.75 accelerator TBBS Zinc oxide 4.0 4.0 4.0 4.0 4.0 4.04.0 4.0 4.0 4.0 4.0 4.0 Evaluation Hardness — — — — — — — — — — — —result (Hs, adjusted to 54 ± 1) EB index 96   100    100    101   105    101    95   104    105    101    100    101    (Target ≥95) RRindex = tan δ 95   102    95   104    106    105    109    102    102   100    97   106    (Target ≥90) Ozone resistance 5   4   4   4   4   4  3+  4   4   4   3+  4   in hot climate (on a 5-point scale, Target ≥3+)Ozone resistance 4   4   4   4   4   4   4   4   4   4   4   4   in coldclimate (on a 5-point scale, Target ≥3+) Brown discoloration 5   4+  4+ 4+  4+  4+  4   4+  4+  4+  3+  4+  at Kobe city (on a 5-point scale,Target ≥3+) White discoloration 5   4+  4+  4+  4   4+  4   4+  4+  4+ 4   4+  at Kobe city (on a 5-point scale, Target ≥3+)

TABLE 3 Comparative Example 1 2 3 4 5 6 7 8 Formulation NR 50   50  50   50   50   50   50   50   (parts by mass) IR — — — — — — — — BR 125   25   25   25   25   25   25   25   BR 2 — — — 25   25   25   25  25   BR 3 25   25   25   — — — — — BR 4 — — — — — — — — BR 5 — — — — — —— — BR 6 — — — — — — — — BR 7 — — — — — — — — BR 8 — — — — — — — — BR 9— — — — — — — — SBR — — — — — — — — Carbon black 1 (N550) 37   37   47  37   37   37   37   37   Carbon black 2 (N220) — — — — — — — — Silica —— — — — — — — Oil 7.0 5.5 12.0  5.5 5.5 5.5 5.5 2.0 Petroleum C5 resin —— — — — — — — Petroleum-derived wax 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Naturally occurring wax — — — — — — — — Surfactant 1 — 1.5 1.5 — — — —6.0 Surfactant 2 — — — — — — — — Surfactant 3 — — — — — — — — Surfactant4 — — — — — — — — Surfactant 5 — — — 1.5 — — — — Surfactant 6 — — — —1.5 — — — Surfactant 7 — — — — — 1.5 — — Surfactant 8 — — — — — — 1.5 —Antioxidant 6C 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Antioxidant TMQ 1.0 1.01.0 1.0 1.0 1.0 1.0 1.0 Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Sulfur 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Vulcanization accelerator TBBS 0.75  0.75  0.75  0.75  0.75  0.75  0.75  0.75 Zinc oxide 4.0 4.0 4.04.0 4.0 4.0 4.0 4.0 Evaluation Hardness (Hs, adjusted to 54 ± 1) — — — —— — — — result EB index (Target ≥95) 100    85   102    86   71   79  84   82   RR index = tan δ (Target ≥90) 100    95   82   92   84   86  93   85   Ozone resistance in hot climate (on a 5-point scale, Target≥3+) 3   3+  2+  3   2+  2+  2+  4   Ozone resistance in cold climate(on a 5-point scale, Target ≥3+) 3   4   4   3   3+  3+  3+  4   Browndiscoloration at Kobe city (on a 5-point scale, Target ≥3+) 3   4+  4  4+  4   4   4   5   White discoloration at Kobe city (on a 5-pointscale, Target ≥3+) 3   4+  4   4   4   3+  4   5   Comparative Example 910 11 12 13 14 15 Formulation NR 50   50   50   50   50   20   50  (parts by mass) IR — — — — — 15   — BR 1 25   25   — — — — 25   BR 225   25   — — — — 25   BR 3 — — — — — — — BR 4 — — — — — 65   — BR 5 — —50   — — — — BR 6 — — — — — — — BR 7 — — — 50   — — — BR 8 — — — — 50  — — BR 9 — — — — — — — SBR — — — — — — — Carbon black 1 (N550) 37   37  39   39   39   — 47   Carbon black 2 (N220) — — — — — 13   — Silica — —— — — 5   — Oil 6.0 6.0 — — — — 15.0  Petroleum C5 resin — — 5.0 5.0 5.03.0 — Petroleum-derived wax 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Naturallyoccurring wax — — — — — — — Surfactant 1 1.5 1.5 1.5 1.5 1.5 1.5 1.5Surfactant 2 — — — — — — — Surfactant 3 — — — — — — — Surfactant 4 — — —— — — — Surfactant 5 — — — — — — — Surfactant 6 — — — — — — — Surfactant7 — — — — — — — Surfactant 8 — — — — — — — Antioxidant 6C 0.5 9.0 4.04.0 4.0 4.0 3.0 Antioxidant TMQ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Stearic acid1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulfur 2.0 2.0 2.0 2.0 2.0 2.0 2.0Vulcanization accelerator TBBS  0.75  0.75  0.75  0.75  0.75  1.20  0.75Zinc oxide 4.0 4.0 4.0 4.0 4.0 4.0 4.0 Evaluation Hardness (Hs, adjustedto 54 ± 1) — — — — — — — result EB index (Target ≥95) 92   107    96  85   72   70   100    RR index = tan δ (Target ≥90) 100    89   87  81   110    115    78   Ozone resistance in hot climate (on a 5-pointscale, Target ≥3+) 2   5   4   4   4   3+  2+  Ozone resistance in coldclimate (on a 5-point scale, Target ≥3+) 2   5   4   4   4   4   4  Brown discoloration at Kobe city (on a 5-point scale, Target ≥3+) 5  3   4+  4+  4+  4   4   White discoloration at Kobe city (on a 5-pointscale, Target ≥3+) 5   3   4+  4+  3+  3+  4  

TABLE 4 Amount of normal alkane according to carbon number (in wax)Petroleum-derived wax (Trial product) Carbon number 19 0.1 of normalalkanes 20 0.29 21 0.68 22 1.31 23 2.32 24 3.3 25 4.14 26 4.38 27 4.5828 3.92 29 3.92 30 3.61 31 4.16 32 4.13 33 4.59 34 4.32 35 4.7 36 4.4737 4.31 38 3.71 39 3.3 40 2.88 41 2.48 42 2.09 43 1.7 44 1.42 45 1.13 460.9 47 0.72 48 0.56 49 0.42 50 0.35 51 0.23 52 0.17 53 0.12 54 0.09 550.06 Amount of C20 to C32 normal alkanes (% by mass) 40.7 Amount of C33to C44 normal alkanes (% by mass) 40.0 Amount of C45 to C47 normalalkanes (% by mass) 2.8 Amount of C48 and higher normal alkanes 2.0 (%by mass) Amount of iso-components (% by mass) 14.5

The rubber compositions of the examples containing: a rubber componentincluding at least one selected from the group consisting of a specificnon-oil extended polybutadiene rubber that has a cis content of 95% bymass or more and a Mooney viscosity (ML₁₊₄, 100° C.) of 50 or more andan oil extended polybutadiene rubber that has a cis content of 95% bymass or more and a weight average molecular weight (Mw) of 420,000 orhigher, the rubber component including a predetermined amount of dienerubber; a predetermined amount of a phenylenediamine antioxidant; apredetermined amount of a specific nonionic surfactant; and apredetermined amount of carbon black and/or silica, preventeddiscoloration and improved ozone resistance while maintaining orimproving good elongation at break and good fuel economy.

The invention claimed is:
 1. A rubber composition for tires, comprising:a rubber component comprising at least one selected from the groupconsisting of a non-oil extended polybutadiene rubber that has a ciscontent of 95% by mass or more and a Mooney viscosity (ML₁₊₄, 100° C.)of 50 or more and an oil extended polybutadiene rubber that has a ciscontent of 95% by mass or more and a weight average molecular weight(Mw) of 420,000 or higher, the non-oil extended polybutadiene rubberbeing at least one selected from the group consisting of a polybutadienerubber synthesized using a rare earth catalyst and a polybutadienerubber containing 1,2-syndiotactic polybutadiene crystals, the rubbercomposition having an amount of diene rubber of 70 to 100% by mass basedon 100% by mass of the rubber component; 1.0 to 8.0 parts by mass, per100 parts by mass of the rubber component, of a phenylenediamineantioxidant; 0.1 to 5.0 parts by mass, per 100 parts by mass of therubber component, of a nonionic surfactant, the nonionic surfactantbeing at least one selected from the group consisting of polyoxyethylenepolyoxypropylene glycols, polyoxyethylene polyoxypropylene blockpolymers, and polypropylene glycol ethylene oxide adducts, and at leastone of nonionic surfactants represented by Formula (1) or Formula (2)below,

wherein R¹ represents a C6-C26 hydrocarbon group, and d represents aninteger,

wherein R² and R³ are the same or different and each represent a C6-C26hydrocarbon group, and e represents an integer; and 2 to 45 parts bymass, per 100 parts by mass of the rubber component, of carbon blackhaving a nitrogen adsorption specific surface area (N₂SA) of 20 to 200m²/g, and optionally silica, a combined amount of carbon black andsilica being 20 to 45 parts by mass per 100 parts by mass of the rubbercomponent.
 2. The rubber composition for tires according to claim 1,wherein the rubber composition comprises, based on 100% by mass of therubber component, 15 to 70% by mass of the non-oil extendedpolybutadiene rubber or 5 to 50% by mass of a polybutadiene rubbercomponent contained in the oil extended polybutadiene rubber, or whereinthe rubber composition comprises, based on 100% by mass of the rubbercomponent, 15 to 70% by mass of the non-oil extended polybutadienerubber and 5 to 50% by mass of a polybutadiene rubber componentcontained in the oil extended polybutadiene rubber.
 3. The rubbercomposition for tires according to claim 1, wherein the non-oil extendedpolybutadiene rubber has a Mooney viscosity (ML₁₊₄, 100° C.) of 60 ormore.
 4. The rubber composition for tires according to claim 1 furthercomprising a petroleum-derived wax, wherein an amount of thepetroleum-derived wax is 6.0 parts by mass or less per 100 parts by massof the rubber component.
 5. The rubber composition for tires accordingto claim 4, wherein based on 100% by mass of the petroleum-derived wax,a combined amount of C20 to C32 normal alkanes is 25 to 50% by mass anda combined amount of C33 to C44 normal alkanes is 25 to 50% by mass. 6.The rubber composition for tires according to claim 1, which is a rubbercomposition for tire outer layers.
 7. A pneumatic tire, formed from therubber composition according to claim
 1. 8. The rubber composition fortires according to claim 2, wherein the non-oil extended polybutadienerubber has a Mooney viscosity (ML₁₊₄, 100° C.) of 60 or more.
 9. Therubber composition for tires according to claim 2 further comprising apetroleum-derived wax, wherein an amount of the petroleum-derived wax is6.0 parts by mass or less per 100 parts by mass of the rubber component.10. The rubber composition for tires according to claim 3 furthercomprising a petroleum-derived wax, wherein an amount of thepetroleum-derived wax is 6.0 parts by mass or less per 100 parts by massof the rubber component.
 11. The rubber composition for tires accordingto claim 2, which is a rubber composition for tire outer layers.
 12. Therubber composition for tires according to claim 3, which is a rubbercomposition for tire outer layers.
 13. The rubber composition for tiresaccording to claim 4, which is a rubber composition for tire outerlayers.
 14. The rubber composition for tires according to claim 5, whichis a rubber composition for tire outer layers.
 15. A pneumatic tire,formed from the rubber composition according to claim
 2. 16. A pneumatictire, formed from the rubber composition according to claim
 3. 17. Apneumatic tire, formed from the rubber composition according to claim 4.18. A pneumatic tire, formed from the rubber composition according toclaim
 5. 19. A pneumatic tire, formed from the rubber compositionaccording to claim 6.